The interrelation between cyclic GMP and cyclic AMP in cell growth and differentiation has been studied in rat adrenal and testis. Levels of cyclic AMP in adrenal are increased during rapid growth, while cyclic GMP is decreased. In testis cyclic AMP and cyclic GMP concentrations are high in the infantile stage and fall progressively till puberty, at which time cyclic AMP levels increase, while cyclic GMP continues to decrease. Cyclic nucleotide immunocytochemistry has been applied to rat testis during different stages of spermatogenesis. Cyclic AMP is found in Sertoli cells, spermatogonia and cells in the intertubular area. Cyclic GMP is found in all cell types; cyclic GMP is found in nuclear elements, particularly along pachytene chromosomes and in nucleoli. BIBLIOGRAPHIC REFERENCES: Rosman, P.M., R. Agrawal, A.D. Goodman, and A.L. Steiner. Effect of angiotensin on cyclic guanosine monophosphate and cyclic adenosine monophosphate in human plasma. J. Clin. Endo. Met. 42:531, 1976. Charles, M.A., J. Laweck, A.L. Steiner, and G.M. Grodsky. Cyclic nucleotides in pancreatic islets. Tolbutamide and arginine induced insulin release. Diabetes 25:256, 1976.